In many conventional centrifugal blowers, a centrifugal multi-blade fan is provided in a central portion of a scroll casing. The scroll casing includes an air passage in which air blows radially outward due to rotational motion of the centrifugal multi-blade fan. An air-blowing exit is provided at a scroll finish side of the scroll casing and air blows through the exit and out of the blower.
In addition, in many conventional centrifugal blowers, a radius of the scroll casing (scroll radius) increases from a scroll start side (nose portion) toward a scroll finish side of the scroll casing. Thereby, a width of the air passage (dimension of the air passage in the radial direction of the centrifugal multi-blade fan) increases from the scroll start toward the scroll finish side of the scroll casing. Since a cross sectional area of the air passage increases from the scroll start side toward the scroll finish side of the scroll casing, occurrence of stagnation or contraction of air flow in the air passage is reduced. Also, it is possible to increase a flow amount of the air from the scroll start side toward the scroll finish side of the scroll casing. JP-2002-339899A discloses one example of this type of centrifugal blower.
However, these conventional centrifugal blowers can create undesirable noise. More specifically, since the width of the air passage is abruptly reduced from the scroll finish portion toward the scroll start portion of the scroll casing, static pressure between blades at the scroll start side becomes abruptly higher as compared to a static pressure between blades at the scroll finish side (refer to a comparative example 1 in FIG. 8 to be described in greater detail below). Further, noise can be caused by fluctuations of the static pressure between the blades.
In response to this problem, the scroll radius can be enlarged at the scroll start portion to increase the width of the air passage at the scroll start portion, thus avoiding an abrupt reduction in the width of the air passage from the scroll finish portion toward the scroll start portion of the scroll casing. However, simply enlarging the width of the air passage at the scroll start portion results in an expansion of a communicating area between the scroll finish portion and the scroll start portion. As a result, air re-circulation can increase from the scroll finish side (air-blowing exit) portion toward the scroll start (hereinafter refer to this air as recirculation flow) to reduce a blowing pressure, thereby reducing blowing properties. In addition, an increase of the recirculation flow leads to an increase in noise caused by interaction of the recirculation flow and the air blown from the centrifugal multi-blade fan.
In view of the above, there exists a need for a centrifugal blower which overcomes the above mentioned problems in the conventional art.